IRRITABILITY. 497 



It is clear that plant-organs growing in obedience to these 

 internal directive influences will offer a certain resistance 

 to the action of a directive influence acting from without. It 

 is perhaps to this that the occurrence of the latent period 

 (pp. 435, 464) is to be attributed. 



With regard to the combined action of the external direc- 

 tive influences, it was pointed out in a previous lecture (p. 424) 

 that the direction of growth of a plant-organ depends upon its 

 peculiar irritability, and upon a certain balance between the 

 responses given by it to the various directive influences 

 which act upon it. We will now discuss this subject in 

 detail. 



It has been sufficiently shewn in previous lectures that 

 plant-organs are sensitive to a variety of external directive 

 influences, and it has been pointed out (p. 374) that each 

 organ possesses a specific irritability to the action of each 

 such influence. In endeavouring to account for the position 

 assumed by different plant-organs under their several con- 

 ditions of growth, we must bear in mind that they may 

 differ from each other both in the kind and in the degree of 

 their specific irritabilities; and further, inasmuch as we have 

 seen that the response to the action of any given external 

 influence varies with the strength of that influence, we must 

 in all cases enquire into the strength of the directive influences 

 at work. 



We will begin with orthotropism. We have found, namely, 

 that certain organs, which are radially or isobilaterally organ- 

 ised, grow vertically either upwards or downwards under what 

 may be regarded as the normal conditions of their growth; 

 that, for instance, primary shoots and isobilateral leaves grow 

 vertically upwards when they are fully exposed to light, and 

 that primary roots grow vertically downwards in a uniformly 

 moist soil. 



Taking first the case of the shoots and leaves, we find that 

 their orthotropism is the result of their negative geotropism and 

 of their positive heliotropism. The action of the directive 

 influence of gravity upon these organs is clearly demonstrated 

 by the fact that they grow vertically upwards in darkness; and, 



V. 32 



